A measuring arrangement of this type for the nondispersive measurement of the infrared absorption with a fixed absorption measuring section has been known from, e.g., DE 3509532 C2.
In such nondispersive infrared measuring arrangements, the attenuation of the infrared radiation absorbed by the gas to be determined after passing through a predetermined absorption section is an indicator of the gas concentration present.
The relationship between the intensity I at the end of the absorption section and the gas concentration C is described by the Bouguer-Lambert-Beer law: EQU I(C)=I.sub.0 .multidot.exp(-e.multidot.L.multidot.C)
in which
I.sub.0 =intensity at entry; e=extinction coefficient; PA1 L=absorption section; C=gas concentration.
It is important for the high sensitivity of the instruments that a sufficient amount of infrared radiation shall reach the infrared radiation detector in order for the detector noise not to become too intense, and that the measured intensity I(C) shall change as greatly as possible if the concentration C is changing.
It is seen from the equation given above that due to being in the exponent, only little infrared radiation reaches the infrared radiation detector at high concentrations, and the change in the signal as a function of the change in the gas concentration is weak as well.
Consequently, to achieve high sensitivity, the sensitivity of the absorption section L must be adapted to the concentration to be measured, i.e., short sections shall be used for high concentrations and vice versa. Consequently, if the concentration range to be measured varies within a broad range, two sample holders with absorption sections of different lengths are actually needed.